-Weverything should not enable -Wmissing-exported-signatures

The -Weverything compiler flag enables every compiler warning. That includes -Wmissing-exported-signatures, which disables -Wmissing-signatures (With -Wmissing-signatures, every top-level binding without a signature generates a warning, with -Wmissing-exported-signatures, only exported bindings generate a warning).

While technically "every warning" includes -Wmissing-exported-signatures, I don't think this matches how people use -Weverything. At least personally I think of it as "-Wall isn't enough, give me the strictest possible warning settings", but actually -Weverything can have fewer warnings than -Wall because of -Wmissing-exported-signatures.